Method of strengthening glass by plasma induced ion exchanges, and articles made according to the same

ABSTRACT

Certain example embodiments relate to an improved method of strengthening glass substrates (e.g., soda lime silica glass substrates). In certain examples, a glass substrate may be chemically strengthened by creating an electric field within the glass. In certain cases, the chemical tempering may be performed by surrounding the substrate by a plasma including certain ions, such as Li + , K + , Mg 2+ , and/or the like. In some cases, these ions may be forced into the glass substrate due to the half-cycles of the electric field generated by the electrodes that formed the plasma. This may advantageously chemically strengthen a glass substrate on a substantially reduced time scale. In other example embodiments, an electric field may be set in a float bath such that sodium ions are driven from the molten glass ribbon into the tin bath, which may advantageously result in a stronger glass substrate with reduced sodium content.

Certain example embodiments relate to an improved method ofstrengthening glass substrates (e.g., soda lime silica glasssubstrates). In certain examples, a glass substrate may be chemicallystrengthened by creating an electric field within the glass. In certaincases, the chemical tempering may be performed by surrounding thesubstrate by a plasma including certain ions, such as Li⁺, K⁺, Mg²⁺,and/or the like, with such ions being forced into the glass substrate asa function of the half-cycles of the electric field generated by theelectrodes that formed the plasma. In certain example embodiments, anelectric field may be set in a float bath such that sodium ions aredriven from the molten glass ribbon into the tin bath, which mayadvantageously result in a stronger glass substrate with reduced sodiumcontent.

BACKGROUND AND SUMMARY OF CERTAIN EXAMPLE EMBODIMENTS

Soda lime silica glass has many desirable properties in a wide range ofconditions including, for example, good transparency and clarity, highdurability, etc. However, in some cases, the degree of mechanicalstrength of soda lime silica glass may depend upon the presence of (i)flaws in the glass originating at fabrication; and/or (ii) surface flawsthat develop as the surface of the glass corrodes over time, which maycause a so-called “blistering” at the glass surface. These surface flawssometimes may become more significant as the surface area to volumeratio of a piece of glass increases (e.g., for thinner articles ofglass). While sodium dioxide (NaO₂) may be used to help reduce themelting point of glass during its manufacture in certain circumstances,the presence of Na⁺ ions in a glass article (and particularly toward thesurface/near surface area of the glass article) may have a negativeimpact on the chemical and/or mechanical durability of the article insome cases.

For example, in certain cases, Na⁺ ions may cause soda lime silica glassto deteriorate in quality. This may occur in a two-stage process in somecases. The first stage may be an ion-exchange process between H⁺ and/orH₃O⁺ ions from moisture that penetrates the surface of the soda limesilica glass and an alkali metal ion (e.g., Na⁺) that is removed fromthe glass. At this stage, the silica network may remain unchanged, butan alkaline film of or including NaOH and H₂O may form at the glasssurface. In some examples, the film may become increasingly alkaline, asmore moisture penetrates the surface of the glass and reacts with thesodium ions near the surface of the glass. In certain cases, if and/orwhen the pH reaches approximately 9 or higher (e.g., as the film becomesincreasingly basic), the second stage may occur. The second stage mayinclude decomposition of the silica network, which may lead to theformation of nano-cracks that may, in turn, potentially weaken the glasssubstrate.

Furthermore, glass strength may sometimes be controlled by the size ofthe “worst” or most severe defect (e.g., cracks and/or nano-cracks),which may vary from sample to sample. In certain cases, the surfacenano-cracks and/or other flaws in soda lime silica glass (e.g., createdby the reaction between Na⁺ ions and moisture and/or those presentbecause of other factors) may act as “stress concentrators.” In somecases, less stress may be needed for the crack to become bigger, spread,and/or break the glass substrate because these nano-cracks are presentin the first place. Additionally, because these cracks may form near thesurface of the glass (e.g., because of the reaction of sodium ions andmoisture to form an alkaline “coating” near the surface of the glass),the surface of the glass may be particularly prone to damage from loweramounts of stress than normal (e.g., more prone than a crack-freeportion of soda lime silica glass).

In some general cases, the larger the crack, the lower the stressrequired for the crack to propagate. The potentially large number offlaws may cause the glass to fracture more easily than it normallywould. In some cases, unstrengthened glass may fracture at stresses thatare 5, 10, or even 100 times lower than the theoretical strength of theglass, e.g., because of the presence of cracks/nano-cracks. This maymake it difficult to accurately predict the amount of stress a glasssubstrate may be subjected to before the glass will crack, fracture,and/or break. This variability in the strength of glass substrateshaving similar compositions and thicknesses may lead to increasedaccidental breakage and/or increased production costs, e.g., related tohaving to produce a glass substrate of higher strength than necessary tobe “safe.” Indeed, there may be about 25% variability in the stressesthat could cause glass substrates of similar compositions andthicknesses to crack, fracture, and/or break.

In other words, the amount of stress required to cause different glasssubstrates of similar composition and thickness to crack, fracture,and/or break may vary. The amount of stress that causes one substrate tofracture may not fracture a second substrate of similar composition andthickness. In some cases, it may be necessary to increase the thicknessof glass because of the possible combination of low overall strength andhigh strength variability, if glass is to be used at all. Thus, in somecases, the use of soda lime silica glass as an engineering material maybe limited by issues arising from these flaws, such as its brittlefracture behavior, strength variations, and/or its low effectivestrength under normal use conditions.

As the application of thin glass becomes more widespread (e.g., in theelectronics market), factors such as mechanical hardness, resistance tomarring and scratching, as well as thermal stability, become potentiallymore important considerations.

Thus, it will be appreciated that there is a need for improving thesurface and/or near-surface properties of glass so that it may besufficiently durable while maintaining and/or improving its otherdesirable properties (e.g., electronic grade, transparency, etc.).

There currently are several methods that may be employed to strengthensoda lime silica glass, namely, thermal (e.g., mechanical) heattreatment (e.g., heat strengthening and/or thermal tempering) andchemical tempering. Heat treating, for example, typically involvesheating a glass substrate, and cooling the hot surface more rapidly thanthe interior. This creates compressive stress near the surface, which isbalanced and/or offset by tensile stress toward the interior. Chemicaltempering, on the other hand, involves an ion-exchange process. Incertain chemical tempering implementations, larger ions are substitutedfor smaller ions at the surface of the glass. This process is sometimesreferred to as “ion stuffing.” Both techniques may induce a compressivestress in the surface of the glass substrate in some instances.

A stress profile shows the relative amount and type of stress present inthe substrate at various points. FIGS. 1( a)-(b) are illustrative stressprofiles for each of the two techniques described above. Moreparticularly, FIG. 1( a) illustrates a cross-section of a glasssubstrate, showing the residual stress profile caused by thermallystrengthened glass.

As discussed above, heat treating (e.g., heat strengthening and/orthermal tempering) for glass involves strengthening the glass byaltering the stress of the glass. In some cases, thermal temperingmethods will build up and/or increase a residual compressive stressstate at the surface of a glass substrate, and up to a certain depthbelow the surface. This residual compressive stress state isequilibrated and/or balanced by a tensile stress state in at least someof the internal portions of the glass. FIG. 1( a) illustrates thecompressive stress at the surface and/or near-surface portion(s) 100(a)of substrate 1(a), and the tensile stress toward the interior portion101(a) of substrate 1(a). As can be seen from FIG. 1( a), the stress ofinterior portions of a glass substrate that has been mechanicallystrengthened (e.g., heat treated) may be tensile, in certain cases.

FIG. 1( b), on the other hand, illustrates the cross-section of a glasssubstrate that has been chemically tempered. In FIG. 1( b), whilesubstrate 1(b) has compressive stress in areas 100(b) at/near itssurface, the compressive stress does not penetrate as deeply into thesurface, and the tensile stress in the interior portion 101(b) of theglass substrate 1(b) is lower than glass substrate 1(a) that wasthermally tempered. When a glass substrate is chemically tempered, thecomposition of the surface and/or near-surface regions of the substrateis changed. Na⁺ ions may be removed in certain cases, and thus thesilica network of the glass substrate generally will not be assusceptible to damage related to reactions between Na⁺ ions and externalmoisture.

A disadvantage of thermal tempering is that in some cases it may not beused effectively for thin glass (e.g., glass that is approximately orless than 1.5 mm in thickness) because it may cause surface wrinklingand/or warping. Further, thermal tempering may sometimes lose efficacyas the glass gets thinner. In other examples, thermal tempering may notachieve the same level of “temper” as in chemical tempering. However,chemical tempering may take a very long to sufficiently strengthen aglass substrate (e.g., hours as opposed to minutes).

Thus, although thermal and chemical tempering may be advantageous incertain instances, those skilled in the art will appreciate that thereis a need for faster ways to better strengthen a glass substrate (e.g.,for thick and thin substrates).

In certain example embodiments, a method for increasing the strength ofa glass substrate is provided. A plasma is struck using at least oneplasma source and first and second electrodes disposed on opposing majorsurfaces of a glass substrate, wherein the plasma comprises replacementions. The replacement ions are driven into the opposing major surfacesof the glass substrate so as to increase the strength of the glasssubstrate.

In certain example embodiments, a method of using plasma to strengthen aglass substrate comprising sodium ions is provided. A plasma is struckusing at least one plasma source and first and second electrodesdisposed on opposing major surfaces of a glass substrate, with theplasma comprises positive ions. An electric field is used to drive thepositive ions into the at least one major surface of the glass substrateso as to replace at least some of the sodium ions and increase thestrength of the glass substrate.

In certain example embodiments, a chemically-strengthened glass articlecomprising soda lime silica glass is provided. The article comprises atleast one of potassium, lithium and magnesium plasma-implantedreplacement ions in a surface region of the glass article. The surfaceregion extends from a major surface of the glass article to a depth ofat least about 5 microns, more preferably at least about 7 microns, andsometimes at least about 50 microns. At least some of the replacementions have replaced sodium ions such that the glass article has fewersodium ions than a glass article that has not been chemicallystrengthened. The glass article has a strength of at least about 200MPa, and more preferably at least about 400 MPa.

In certain example embodiments, a method for increasing the strength ofa glass substrate is provided. At least one plasma torch comprising atleast first and second electrodes is disposed on at least a first majorsurface of a glass substrate. A plasma comprising replacement ions issprayed through a nozzle of the plasma torch via an applied electricfield between the two electrodes such that the plasma is sprayedproximate the first major surface of the glass substrate. Thereplacement ions are driven into the at least one major surface of theglass substrate so as to increase the strength of the glass substrate.

In certain example embodiments, a method for strengthening a soda limesilica glass substrate is provided. First and second plasma torches orarc jets are disposed on opposing major surfaces of a glass substrate. Aplasma comprising replacement ions is sprayed onto the opposing majorsurfaces of the glass substrate via each plasma torch or arc jet. Thereplacement ions are driven into the first and second major surfaces ofthe glass substrate by virtue of electric fields between the first andsecond electrodes of each plasma torch or arc jet, so as to increase thestrength of the glass substrate.

In certain example embodiments, a method of making a glass substrate isprovided. Opposing major surfaces of a soda lime silica glass substrateare exposed to plasmas containing ions, the soda lime silica glasssubstrate at least initially including 10-20 wt. % Na₂O. Electrodesassociated with the plasma are selectively activated to drive the ions,directly or indirectly, into surface regions of the glass substrate andforce sodium ions out from the surface regions to reduce Na₂O content ofthe glass substrate to less than 10 wt. %.

In certain example embodiments, a method of making a glass substrate isprovided. A plasma is struck in a tin bath section of a float line atleast over a molten glass ribbon, with the plasma acting as a positiveelectrode and the tin bath acting as a negative electrode. Sodium ionsare driven out of the molten glass ribbon and into the tin bath via anelectric field created by the positive and negative electrodes and atleast partially present in the molten glass ribbon. The glass substrateis allowed to be formed, with the glass substrate having less than 20wt. % Na₂O.

In certain example embodiments, a method of making a glass substrate isprovided. A molten glass ribbon is provided in a tin bath portion of afloat line via at least one plasma. Sodium ions are driven out of themolten glass ribbon and into the tin bath so as to reduce the sodiumion-content of the molten glass ribbon. The glass ribbon is maintainedat one or more temperatures high enough such that the glass ribbonremains in a molten state, even as the composition of the glass ribbonchanges, in making the glass substrate.

In certain example embodiments, a strengthened glass substrate comprisesa silicate matrix, wherein the matrix comprises at least some argonatoms and wherein the glass substrate is substantially depleted ofsodium ions and has a strength of at least about 600 MPa, and morepreferably at least about 1000 MPa.

In certain example embodiments, a method of making a glass substrate isprovided. Alumina is driven into molten glass, and sodium is forced outof the molten glass, via at least one plasma including alumina. Themolten glass is pulled in making the glass substrate.

In certain example embodiments, a strengthened glass substrate comprisesless than 10 wt. % Na₂O and at least one type of ion selected from thegroup consisting of potassium, lithium and magnesium ions. At least someof the ions have replaced sodium ions originally present in a glassribbon leading up to the glass substrate. The glass article has astrength of at least about 400 MPa.

These and other embodiments, features, aspect, and advantages may becombined in any suitable combination or sub-combination to produce yetfurther embodiments.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other features and advantages may be better and morecompletely understood by reference to the following detailed descriptionof exemplary illustrative embodiments in conjunction with the drawings,of which:

FIGS. 1( a)-(b) show example stress profiles for mechanically (e.g.,heat) strengthened and chemically tempered glass substrates,respectively;

FIGS. 2( a)-(b) help demonstrate certain chemical tempering principles;

FIG. 3 helps demonstrate how a glass substrate can be chemicallystrengthened by using plasma as an electrode and a replacement ionsource in accordance with certain example embodiments;

FIG. 4 illustrates an enlarged cross-section of a chemicallystrengthened glass substrate according to certain example embodiments;

FIG. 5 helps demonstrate how one or more plasmas may be used to supplyreplacement ions when chemically strengthening a glass substrate inaccordance with certain example embodiments;

FIG. 6 illustrates an enlarged cross-section of a chemicallystrengthened glass substrate, including an ion gradient, according tocertain example embodiments;

FIG. 7 helps demonstrate how a glass substrate can be chemicallystrengthened using a plasma along a float line in accordance withcertain example embodiments;

FIG. 8 is a graph of example electrode voltage versus time, which may beused in accordance with certain example embodiments; and

FIG. 9 illustrates a graph of voltage over time and the correspondingeffects on the sheath surrounding the glass substrate, according tocertain example embodiments.

DETAILED DESCRIPTION OF CERTAIN EXAMPLE EMBODIMENTS

As explained above, it may be desirable in certain instances to makethin glass substrates of improved durability, e.g., that are “electronicgrade.” In addition, it may be desirable in certain cases to makeimproved glass starting from an imperfect stock of soda lime silicaglass. Indeed, it sometimes may be more cost-effective to treat sodalime silica glass in a manner that renders it more durable and/orstrong, rather than to manufacture an inherently stronger glass such asborosilicate glass and/or the like. In order to help overcome thecomparatively lower strength of soda lime silica glass, and make itthinner, stronger, and/or electronic grade, it may be desirable to altercertain properties of the surface and/or near-surface region(s) of aglass substrate. Thus, for example, sodium ions that maydisadvantageously react with moisture to degrade a glass substrate maybe reduced and/or replaced in certain example embodiments.

FIGS. 2( a) and 2(b) help demonstrate certain chemical temperingprinciples. As explained above, the general concept of chemicaltempering involves removing and/or replacing some of the sodium ions inthe glass substrate (particularly those in the surface and/ornear-surface region) with other replacement ions. The replacement ionsmay be larger than and/or less reactive than the sodium ions. In somecases, the replacement ions may include potassium, lithium, and/ormagnesium. In certain example embodiments of this invention, replacementions may comprise these ions and/or other divalent ions, multivalentions, alkali metals, silver, and/or other appropriate materials. Incertain cases, the replacement of sodium ions may also change the stressprofile near the surface of the glass substrate.

FIGS. 2( a) and 2(b) also help illustrate the ion exchange between thesurface and/or near-surface area of the glass substrate and the ions inthe molten salt bath. FIG. 2( a), for instance, shows sodium ions 11 ina portion of glass substrate 1. Replacement ions 10 (potassium ions 19may be used as replacement ions 10, in certain cases). Potassium (forexample) ions 19 are shown outside of the glass substrate, at leastinitially. Potassium ions 19 are in the molten salt bath, as issubstrate 1 (though only a portion of the salt bath and a portion of thesubstrate are illustrated). FIG. 2( b) illustrates how sodium ions 11 inglass substrate 1 are replaced with potassium ions 19. The arrows inFIG. 2( b) show a sodium ion 11 leaving the surface/near-surface regionof substrate 1, and indicate that sodium vacancies 15, where sodium ions11 once were, may be replaced by potassium ions 19. In some cases, ions19 may be an ion/compound besides potassium (e.g., another salt). FIGS.2( a) and 2(b) are simply illustrative of how smaller (or in otherexample embodiments, perhaps more reactive rather than smaller) ions 11are replaced with ions 19 in substrate 1, when the substrate is in amolten salt bath.

The ion exchange process (e.g., as illustrated in FIG. 2( b), forexample) may be diffusion-limited in certain instances. The ion exchangeprocess performed in a molten salt bath also typically is a slowprocess. Accordingly, it may take a long time for a sufficient exchangeof ions to take place.

The ion exchange process generally involves a mutual ion diffusionmechanism so as to meet the principle of charge neutrality (e.g., thesum of positive charges generally must equal the sum of negativecharges), since no electron current is involved in some cases. In otherwords, if the Na⁺ ions in the substrate are replaced by ions in themolten salt bath through diffusion forces only, the process may takemany hours. The replacement of a substantial amount of Na⁺ ions via amolten salt bath, with no additional forces at work aside fromdiffusion, may take an extremely long time.

An equation that may be used to express the depth to which thereplacement ions may permeate the glass substrate (L_(exc)) may beexpressed as a function of the diffusivity of a material (e.g.,diffusion coefficient D), which is the degree to which a material allowspassage of a gas or fluid, and the temperature of the glass substrate T,as well as the time t in which the substrate is immersed in the bath.

For instance, L_(exc)≅√{square root over (D_(k)(T)*t)}≅50 μm for a 400MPa temper state (e.g., where the strength of the chemically temperedglass is such that compressive stress on the surface of the glass isapproximately 400 MPa). However, in other examples, a 400 MPa strengthmay be reached with depths of about 7 microns.

The lower limit of time (t) is typically temperature (T) dependent. Inother words, the time it takes to sufficiently strengthen the glasssubstrate via an ion exchange in a molten salt bath may decrease as thetemperature increases. However, extremely high temperatures may benecessary even to achieve the aforesaid temper strength in four hours.Although four hours is a long time, at less extreme temperatures, insome instances, it may take up to 16 (or even more) hours for asufficient strength to be reached. For instance, in some cases,t_(upper)≅L²/D_(k)≅4 hours. This may be limited by Fick's laws which,for example, relate to the direction of diffusive flux with respect toconcentration, and how diffusion causes the concentration to change withtime.

By chemically tempering a glass substrate in a molten salt bath, a glasssubstrate may reach a strength of about 400 MPa. However, as can be seenfrom the above, it may take approximately four hours for replacementions to reach an implanted depth of 50 microns at high temperatures, andup to 16 hours or longer at lower (e.g., more “reasonable”)temperatures. Although the ions may be implanted to shallower depths(e.g., about 7 microns), using a molten salt glass to achieve a strengthof around 400 MPa may still take hours. If the desired strength isgreater than 400 MPa, and/or the desired depth of implantation is 100microns or greater, the process of strengthening glass via a molten saltbath may take 16 hours or more. Accordingly, in certain scenarios, thelength of the time that it takes to chemically strengthen a sheet ofglass using ions in a molten salt bath may not be ideal and/or feasiblefor larger-scale production.

As alluded to above, certain strengthening techniques may involve anion-exchange process that operates via a physical mechanism; e.g. thethermal diffusion of ions. However, in some cases, an electric field maybe superimposed, e.g., on the glass substrate. In certain example cases,the presence of an electric field in the salt bath and/or glasssubstrate may produce an index change by altering the glass density,stress, and/or mean polarizability.

In certain instances, the application of an electric field may increasethe velocity and/or speed of the replacement ions in the salt bath, andmay decrease the time necessary for sufficient replacement (and thusstrengthening). This is believed to be a result of the application of afield-effect assisted ion exchange that may provide an additional forcefor the transport of ions from a molten salt. Thus, ions that otherwisemay be difficult to exchange by random, thermal motion (e.g., divalentions and/or other larger, heavier ions) may thus be used to replacesodium in glass, in cases where an electric field is used.

More specifically, ions used in a molten salt bath to replace sodiumions in a glass substrate may move only because of the diffusion current(e.g., manifested via thermal and/or density gradients) that resultsfrom the random Brownian motion of charge carriers independent ofelectrical stimulus. On the other hand, if an electric field is applied,the ions may move because of the drift current. Drift current is theelectric current, or movement of charge carriers, that is caused by theapplied electric field, often stated as the electromotive force over agiven distance.

Thus, providing replacement ions and producing an electric field toassist the speed at which the replacement ions penetrate the glasssurface and replace the sodium ions is desirable and advantageous incertain example instances. However, it sometimes may be difficult togenerate an electric field of a sufficient magnitude to substantiallyspeed up the replacement process, particularly because electrodes togenerate an electric field in and/or near the glass substrate may needto be placed in the salt bath and may need to directly contact thesurface of the glass substrate. In order for such contact to besuccessful and/or possible, the glass substrate may need to be in thesolid, rather than molten, state. On the other hand, the glass may needto be maintained in its molten state so as to permit a more efficientexchange rate of sodium ions and replacement ions. Thus, the use of anelectric field of a sufficient magnitude may not be possible when theglass substrate is in a molten state; and similarly, the speed/velocityof the replacement ions (and thus the overall chemical temperingprocess) may require that the glass is in a molten state. Accordingly,these two scenarios for small increases in chemical tempering speed maybe mutually exclusive.

In some cases, using a glass in the molten state permits ions to beexchanged efficiently, e.g., because of their high mobility when theglass is in that state (e.g., as explained by the Einstein relation).Two example cases of the relation are:

$D = \frac{\mu_{q}k_{B}T}{q}$(diffusion of charged particles), and

$D = \frac{k_{B}T}{6{\pi\eta}\; r}$(as in the “Einstein-Stokes equation” for diffusion of sphericalparticles through liquid with low Reynolds number),where:

-   -   D is the diffusion constant,    -   q is the electrical charge of a particle,    -   μ_(g) is the electrical mobility of the charged particle, or the        ratio of the particle's terminal drift velocity to an applied        electric field,    -   k_(B) is Boltzmann's constant,    -   T is the absolute temperature,    -   η is viscosity, and    -   r is the radius of the spherical particle.

$D = {\frac{\mu_{q}k_{B}T}{q} = {{\frac{k_{B}T}{6{\pi\eta}\; r}\mspace{14mu}{so}\mspace{14mu}\mu_{q}} = {\frac{q}{6{\pi\eta}\; r}.}}}$Thus, as can be seen from the above diffusion-related equations, as theviscosity of a material decreases (e.g., as the material becomes lessviscous), the electrical mobility of a charged particle will increase.Accordingly, when the glass is in a molten state, the viscosity will belower, so the charge mobility will be higher. Thus, when a glasssubstrate is in the molten state, the ion-exchange process may befaster. An electric field, however, may further increase the rate of ionexchange. Unfortunately, as alluded to above, in order to maintain anelectric field within and/or near the glass substrate, electrodes mayneed to be physically placed on the glass substrate in certainsituations. In order for this to be possible, the glass may need to bein a solid state.

Using an electric field (e.g., in the order of 100 V/cm) with a glasssubstrate in a molten salt bath may permit ions to reach a speed of fromabout 1 to 4 m/s in some instances. In some cases, ions at this speedmay penetrate about 10 microns into the glass substrate. However, incertain cases, it may be desirable for ions to penetrate farther than 10microns into the glass substrate. Moreover, it may not always beconvenient and/or feasible to maintain a glass substrate in a moltenstate.

For example, in certain example ion exchange processes, the glass may bein a solid state (e.g., T<T_(g), where T_(g) is the temperature at whichthe glass becomes molten), and is immersed in a salt bath. To achieveapproximately a 100 micrometer ion exchange depth may take as long as 5,10, or 16 hours, or even longer (depending on the temperature).

In certain instances, to set up an electric field on the order of about100 V/cm (e.g., 10,000 V/m), physical electrodes may be placed in themelt (molten bath) while the glass is in a solid state. Again, incertain instances, placing physical electrodes in a molten salt bathwhile the glass is in a solid state may present an obstacle to applyinga drift current (e.g., the electric current, or movement of chargecarriers, which is related to the applied electric field, often statedas the electromotive force over a given distance) to expedite ionexchange. In other words, to sufficiently apply an electric field, theglass may be in a solid state. Conversely, the ions will move fasterwhen the glass is in a molten state, because the charge mobility isinversely proportional to viscosity, in certain cases. Furthermore, insome cases, DC electric field-assisted ion exchange may only provideexchange to one surface of the glass.

It therefore will be appreciated by those skilled in the art that thereexists a need for a more efficient method of chemically tempering aglass substrate. It may be desirable to find a method to moreefficiently replace sodium ions with replacement ions utilizing anelectric field, and/or when the glass is in a solid state. Furthermore,it would be advantageous to reduce the time scale necessary forperformance of chemical strengthening.

It has advantageously been found that the techniques disclosed hereinmay permit the ionic make up of the surface and/or near-surface region(e.g., a thin, epitaxial layer) of a glass substrate to be tuned and/ormodulated significantly faster than previous methods of mechanicaland/or chemical tempering. In certain exemplary embodiments, thecomposition of the surface and/or near-surface region of a soda limesilica glass substrate may be tuned and/or modulated on a time scalethat is at least one order of magnitude lower than other methods. Incertain example embodiments, a glass substrate may be strengthened in amatter of minutes rather than hours (or even days).

In the molten salt ion exchange described above, the molten saltsupplies the replacement ions for the sodium ions in the glass. However,certain example embodiments relate to methods and/or techniques ofcreating a high and substantially uniform electric field in and/or atthe surface of a glass substrate in different “state scenarios” (e.g.,solid, molten, etc.). As described above, in some cases, creating a highelectric field may require metal electrodes to physically be placed in amolten salt bath. These techniques may be limited in terms of largeareas of glass, and may also be limited by the state of the glass.

By contrast, in certain example embodiments, it has advantageously beenfound that plasma may be used as an electrode to set up a time-varyingelectric field within the glass. In certain cases, an electric field(e.g., AC, DC, RF, etc.) may be superimposed to drive fasterion-exchange strengthening using different techniques to strengthen theglass more quickly. In some cases, the electric field may have a netvarying modulated basis. In a first example embodiment, replacement ionsmay be introduced to a glass substrate by vaporizing the replacementions in a plasma that serves the purpose of electrodes, by setting up anelectric field in and/or around the glass substrate. The glass substrateto be chemically tempered can be visualized as being immersed within thecapacitively coupled or glow discharge or electrostatic discharge plasmafor example, as shown in FIG. 3. A chemically strengthened glasssubstrate produced in accordance with certain example embodiments isshown in FIG. 4, which is described in further detail below.

The plasma may serve several purposes according to different exampleembodiments. For example, the plasma may heat up the glass, provide asource of replacement ions to reach the glass surface, and/or provideelectrons to complete the displacement current. In certain exampleembodiments, the glass substrate may not need to be heated. Instead, theplasma may operate to increase the temperature of the substrate,particularly in the surface and/or near-surface regions of the glasssubstrate. In some cases the glass may act as a capacitor. The plasmamay also act in a manner such that it capacitively couples to the glass.The friction between the positive ions and the electrons in the plasmanear the glass substrate may cause the glass temperature to rise, incertain cases. The plasma may heat the glass substrate via ohmic lossesin certain examples.

FIG. 3 illustrates glass substrate 1 in chamber 50, with upper electrode30 and lower electrode 32. In certain example instances, upper and lowerelectrodes 30 and 32 may operate with radio frequency (RF) and/oralternating current (AC) voltage. The frequency and voltage of theelectrodes may be of any suitable type according to other exampleembodiments (such as DC, for example), however. Plasma source 60 mayprovide chamber 50 with a plasma precursor 62 (e.g. an inert gas; airwith an entrailment of vaporized ionic species based on salt chemistriesof certain monovalent, divalent and/or multivalent species (e.g., Li⁺,K⁺, and/or Mg²⁺ based compounds; and/or the like). Electrodes 30 and 32may be used to create an electric field which, in turn, will strike aplasma from the plasma precursor 62. The pressure of chamber 50 may beat atmospheric pressure, on the order of atmospheric pressure, and/or atsub-atmospheric pressure in different example embodiments. For example,the pressure may be less than or equal to 760 Ton (1 atm), less than orequal to 380 Ton (0.5 atm), and possibly even about 500 millitorr orless in different example implementations. In certain exampleembodiments, the chamber may be at or near vacuum, although otheratmospheric pressures are possible as indicated above.

The atmosphere may further include other elements in different exampleembodiments. For example, the atmosphere may include one or more inertgases, such as argon, krypton, neon, and the like.

In certain example embodiments, the electrodes may create an electricfield that, in turn, may create a plasma 64 from the plasma precursor62. In certain example embodiments, an electric field may be present inthe glass substrate. The plasma precursor 62 may include salt speciessuch as Li⁺, K⁺, and/or Mg²⁺ in certain example embodiments. In certainexample embodiments, the efficiency of producing a plasma may beincreased or maximized at or near the Stoltow point, at which asubstantially uniform, large-volume plasma is realized. The Stoltowpoint is the point at which ionization happens with a reduced energyrequirement. In certain cases, the power approximately varies with thecube of the applied voltage, but the induced (current) flow cansaturate.

The glass substrate may be located between the upper and lowerelectrodes (and thus, within the plasma), in certain examples. To enablean increased flexibility in treatment conditions, the electrodes 30 and32 may advantageously be designed so as to move independently in xand/or y (and possibly even z) directions so as to permit changes in theplasma field as needed during processing. For instance, the electrodes30 and 32 may scan linearly or non-linearly in a computer-controlledpattern across the substrate 1 in x and/or y directions. Because of thedielectric nature of a glass substrate, the glass substrate may act asan insulator. The presence of the glass substrate in the plasma maycreate a positively charged sheath at and/or above the upper and belowthe lower surfaces of the glass substrate.

In certain example embodiments, the ions may be deposited as a layer onthe glass surface and/or be driven directly or indirectly into the glassby the net field and the (possibly alternating) current. In certainexample embodiments, in a plasma ion exchange, a thin film of at leasttemporarily deposited replacement ions (Li⁺, K⁺, Mg²⁺, etc.) may supplythe replacement ions with the aid of an applied net electric fieldsustained by the plasma sheath on one or both sides of the glass. Incertain cases, this “thin film” may be an aspect of the plasma, and nota physical film deposited on the glass substrate. In other exampleembodiments, however, a thin film of or including the aforesaidreplacement ions may be formed on one or both sides of the glasssubstrate to further facilitate the ion exchange/ion stuffing process.

In certain examples, in the thin film technique, mixed ions such asK₂SO₄, Li₂SO₄, MgSO₄, and/or the like, may form the thin film. Indifferent cases, sulfate salts, halide salts, and/or other types ofsalts may be used to supply the replacement ions. The thin-film ionexchange process may start with the production of a thin coating (forexample, 100-150 nm in thickness) of ionic material on one or both sidesof a glass substrate, in certain cases. In further examples, the glassmay have been cleaned and/or polished prior to cleaning. In otherexample embodiments, as indicated above, the thin film of replacementions may simply include the positive ions from the sheath that forms oneach side of the substrate.

In certain example embodiments, the sodium ions may be driven out, andthe replacement ions may fill the vacancies left by the sodium ions. Insome cases, the replacement ions may advantageously be bigger than thesodium ions (e.g., and induce a compressive stress to strengthen theglass substrate) and/or may be less reactive than the sodium ions suchthat the replacement ions are less likely to react with moisture and thelike in the surface/near-surface region of the glass substrate asreadily as the sodium ions would.

The ion exchange may be performed by applying a varying electric field,e.g., that is in some cases asymmetric enough to produce a net field offrom about 20 to several hundred (or even higher) V/mm. For example, thenet electric field may be from about 20 to 500 V/mm (2×10⁴ to 5×10⁵V/m), more preferably from about 50 to 300 V/mm (5×10⁴ to 3×10⁵ V/m),and most preferably from about 100 to 200 V/mm (1×10⁵ to 2×10⁵ V/m).

In certain example embodiments, the net electric field may produce a fewmicroamperes. In some cases, this may drive the ions from the thin filmand/or the sheath into the glass. After the ion exchange, residual ionsremaining on the glass surface may be removed, e.g., by washing. Incertain example embodiments, other ions such as silver may be used tomodulate the refractive index of the glass substrate. Other ions may beused in the plasma in different example embodiments.

In certain exemplary embodiments, where alternating current (AC) isused, the AC may permit neutralization by electrons in the plasma. Inthese examples, the electric field may keep the ions moving into theglass which may, in turn, help prevent ions from being reduced and/orforming nano-crystals at the surface of the glass. In certain examples,the net field may prevent agglomeration and haze. In some cases, sodiummay also be leached in the plasma by reacting with anionic species.

In further examples, the positive ions in the plasma near the glasssubstrate's major surfaces may be electrically driven by the electricfield into the surface of the glass substrate during each of the halfcycles of the respective electrodes (e.g., AC electrodes). In otherwords, in certain example embodiments, glow discharge (e.g., atatmospheric pressure) using a dielectric barrier discharge, may inducefluid flow. Further, in some cases (e.g., when AC is used), there may betwo processes in the two half-cycles of the electrical drivingoperation. In certain examples, this may be related to the difference inmobility between faster electrons and slower (positive) ions, and thegeometric configurations of the actuator insulator and electrodes.

In certain example embodiments, the first half-cycle may becharacterized by the deposition of the slower ion species (e.g., K⁺,Li⁺, Mg²⁺, etc.) on a first major surface of the glass substrate (e.g.,with the glass substrate acting as an insulator). The second half-cyclemay be characterized by the deposition of the electrons at a fasterrate. Once the energetic (positive) ions to be exchanged fall on theglass surface, they may be driven into the glass surface by the electricfield in the sheath, with a high drift speed. In certain exampleembodiments, the drift speed may be in the order of about 10 m/s. Thedrift speed may be at least about 1 m/s, more preferably at least about5 m/s, even more preferably at least about 10 m/s, and most preferablyat least about 25 m/s, according to different example embodiments. Insome cases, a power-law dependence on the voltage for the resultingforce may be observed in the glass. This may indicate that a largerforce can be generated by increasing the amplitude in certain examples.

In other example embodiments, the effectiveness of the actuator may beenhanced using several techniques. A first way to enhance theeffectiveness of the actuator is to increase the peak value of theperiodic force generation. A second way to enhance the effectiveness isto increase the asymmetry between the voltage half-cycles. In certaininstances, one and/or both of those techniques may be utilized. Thefirst example approach for enhancing the effectiveness of the actuator,increasing the peak value of the periodic force generation, may compriseincreasing the size of the lower electrode 32, increasing the appliedvoltage, and/or increasing the dielectric constant, in certain exampleembodiments. The second example approach for enhancing the effectivenessof the actuator, increasing the asymmetry between voltage half-cycles,may comprise decreasing the frequency of applied voltage, in otherexample embodiments. However, the complex interplay between the abovefactors may determine the actuator performance in driving ions at/intothe glass surface(s). In certain example embodiments, sodium vacanciesmay be created, and those vacancies may then be filled by replacementions (e.g., K⁺, Li⁺, Mg⁺², etc.)

The following equations describe certain properties of the ions beingdriven into the surface of the glass substrate. For example, an equationfor boundary conditions, e.g., for the current continuity at theinterface (e.g., between the glass surface and the plasma), may be:

${\frac{\mathbb{d}\left( {ɛ_{gas}E_{gas}} \right)}{\mathbb{d}t} + \frac{e}{ɛ_{o}\left( {{n_{i}V_{i}} - {n_{e}V_{e}}} \right)}} = \frac{\mathbb{d}\left( {ɛ_{glass}E_{glass}} \right)}{\mathbb{d}t}$where ∈ represents permittivity (for gas and then glass) and Erepresents electric field (in the gas then in the glass), e is Euler'snumber, ∈_(o) is the electric constant that relates the units forelectric charge to mechanical quantities such as length and force, n_(i)is the density of the ions, n_(e) is the density of the electrons, v_(i)is the velocity of the ions, and v_(e) is the velocity of the electrons.

In certain example embodiments, once the replacement ions (e.g., K⁺,Li⁺, Mg²⁺, etc.) reach the glass surface, they may be driven into theglass at the consecutive half-cycles by the drift term in the Gradfunction in the sheath of the plasma. An energy function of electricfield E that may creates more sodium vacancies that are then filled bythe positive replacement ions is described below.

$\frac{\delta\; f}{\delta\; t} = {\nabla_{\upsilon}{\cdot \left( {f{\nabla_{\upsilon}\frac{\delta\; E}{\delta\; f}}} \right)}}$

For some Energy Functional E: f→E(f)

In certain example embodiments, the gradient flow may reduce entropy. Ageneralized formula for nano-porous media (e.g., glass with sodiumvacancies may be:

$\frac{\delta\rho}{\delta\; t} = {{\Delta_{x}\rho^{m}} + {\nabla_{x}{\cdot \left( {\rho\; x} \right)}}}$

In certain example embodiments, the ions may be driven into the glasssubstrate to a depth of from about 1 to 1,000 microns, preferably atleast about 5 microns, more preferably the ions are implanted to a depthof at least about 7 to 15 microns, even more preferably from about 30 to900 microns, and most preferably from about 50 to 200 microns. However,in some cases, the desired strength may be accomplished with ionsimplanted to a depth of only about 5 to 15 (e.g., 7) microns. In certainexample embodiments, the thickness of the glass substrate may be relatedto the depth of implantation necessary to achieve a particular strength.

The depth to which the ions are driven into the glass substrate may beachieved on the first “pulse” from a half-cycle, or may be the result ofmultiple pulses, e.g., with each pulse driving the ion further andfurther into the glass substrate. In certain example embodiments, theelectrodes 30 and 32 may be alternatively actuated, e.g., so that moreand more ions are stuffed into the substrate faster than they canescape. In certain example embodiments, the implanted ions may have asubstantially normal Gaussian, Poisson, or other distribution.

FIG. 4 illustrates an enlarged cross-section of a chemicallystrengthened glass substrate according to certain example embodiments.Replacement ions 10 have replaced at least some of the sodium ions 11 inthe surface and/or near surface regions of chemically strengthenedsubstrate 1′. Replacement ions 10 may include potassium, lithium,magnesium, other alkali metal ions, silver, argon, and/or the like. Atleast some sodium ions 11 may remain in the interior portion 101′.However, in certain example embodiments, they are at a great enoughdistance from the surface region of the glass substrate so that theypresent less of a threat to the strength and/or durability of the glasssubstrate.

In certain example embodiments, the chemical strengthening according toexample embodiments described herein may produce an increase in thestrength of the glass to at least about 200 MPa, more preferably atleast about 400 MPa, and most preferably at least about 600 MPa. Inother example embodiments, the resulting strength of a glass substratethat is chemically tempered utilizing plasma may have a strength of fromabout 200 to 800 MPa, more preferably from about 400 to 600 MPa, andmost preferably a strength of at least about 400 MPa, in differentexample embodiments.

This increase in strength may be accomplished in from about 30 secondsto 30 minutes, more preferably about 1 to 20 minutes, and mostpreferably in about 2 to 10 minutes, with an example amount of timebeing about 5 minutes, but possibly even less. However, the glass may bestrengthened for more or less time according to different exampleembodiments.

In some cases, salts based on halides may produce clouding on a glasssurface. However, halide salts may be used in connection with certainexample embodiments. For instance, divalent and/or multivalent halidesalts may be used in the plasma in certain embodiments. In some cases,binary and ternary sulfate salts may advantageously produce clearcrystallized surface layers of beta quartz solid solution. The salts ofsulfates (e.g., K₂SO₄, Li₂SO₄, etc.), and/or MgSO₄, nitrates, halogens(halides), and/or other precursors including potassium, lithium, and/ormagnesium may also be used certain examples. Other alkali metals may beused in other embodiments. The precursor materials and/or replacementions are not so limited and other materials may be used in differentexample embodiments.

FIG. 5 helps demonstrate how one or more plasmas may be used to supplyreplacement ions when chemically strengthening a glass substrate inaccordance with certain example embodiments. The FIG. 5 exampleembodiment is somewhat similar to the FIG. 4 example embodiment. In FIG.5, a plasma torch (e.g., with an arc jet) is used as the plasma source.Plasma torches/sources are shown schematically and designated withreference numerals 60′ and 60″ in FIG. 5. Example plasma torches aredescribed in U.S. Pat. Nos. 5,998,757 and 6,329,628, which are herebyincorporated by reference. However, any plasma torch and/or plasma arcjet may be used in connection with different example embodiments.

In certain example embodiments, e.g., such as in FIG. 5, the chemicalstrengthening system may include electrodes on one or both sides of theglass. FIG. 5 illustrates first rear and front electrodes 300 and 302 ona first major surface of the glass substrate. Second rear and frontelectrodes 304 and 306 are on the opposing major surface of the glasssubstrate in the FIG. 5 example embodiment. In other exampleembodiments, however, more or fewer electrodes may be used on one orboth sides of the substrate. FIG. 5 also illustrates plasma precursors62 and 62′ and plasmas 64 and 64′ being sprayed proximate the majorsurface(s) of the glass substrate 1.

In certain example embodiments, the net electric field may be from about20 to 500 V/mm (2×10⁴ to 5×10⁵ V/m), more preferably from about 50 to300 V/mm (5×10⁴ to 3×10⁵ V/m), and most preferably from about 100 to 200V/mm (1×10⁵ to 2×10⁵ V/m). In certain examples, the pressure at whichthe plasma torch is used may be atmospheric pressure, on the order ofatmospheric pressure, and/or at sub-atmospheric pressure in differentexample embodiments. The example atmospheres and/or pressure identifiedabove also may be used in connection with the FIG. 5 example embodimentor embodiments similar to it.

In certain example embodiments, as the glass substrate moves through theopening between the electrodes, the ions from the plasma will be driveninto the surface areas of each major surface of the glass substrate. Incertain example embodiments, plasma torches may be provided on each sideof the glass article to be chemically tempered (e.g., one and/or morethan one on each side). In other example embodiments, a plasma source(e.g. plasma torch) may be provided on only one side of the glasssubstrate. The salts of sulfates, nitrates, halogens (halides), and/orother precursors including potassium, lithium, and/or magnesium may besprayed into the injector stage of the plasma torch in certain exampleembodiments. However, other alkali metals may be used in otherembodiments. The precursor materials for the plasma and/or replacementions used in this embodiment may be the same as those used in otherembodiments described herein. Further, the atmosphere may furthercontain Argon or other suitable inert gas(ses).

In certain example embodiments, once vaporized, the ions may beaccelerated through the nozzle of the plasma torch by an appliedelectric field between the two electrodes. The high temperature of thetorch may vaporize the material and spray it onto the glass in certainexample embodiments. The high electric field may permit the ions to bedriven into the glass surface in the order of 100 micrometers in aboutfive minutes or less in some cases.

For instance, the ions may be driven into the glass substrate to a depthof from about 1 to 1,000 microns, preferably at least about 5 microns,more preferably the ions are implanted to a depth of at least about 7 to15 microns, even more preferably from about 30 to 900 microns, and mostpreferably from about 50 to 200 microns with an example depth beingabout 100 microns below the surface of the glass substrate. However, insome cases, the desired strength may be accomplished with ions implantedto a depth of only about 5 to 15 (e.g., 7) microns. The depth to whichthe ions are driven into the glass substrate may be achieved on thefirst “pulse” from a half-cycle, or may be the result of multiplepulses, e.g., with each pulse driving the ion further and further intothe glass substrate. In embodiments where two or more plasma torches areincluded, the plasma torches may be activated together (e.g., at thesame or substantially the same time), in an alternating pattern, or insome other computer-controlled fashion.

In certain example embodiments, this plasma torch embodiment may or maynot involve ion implantation, as the amplitude of voltage involved mayor may not be high enough to implant ions at a depth of at least about100 microns, which in certain example embodiments may be desirable whenattempting to strengthen the glass. In certain cases (e.g., whereimplanting ions is not feasible or possible), the wave form may be usedto progressively push the ions into the glass on the positive pulse, sothat with each pulse, the ions are driven a bit further into the glasssubstrate until they reach an acceptable depth (e.g., at least about 100microns). In some cases, corrective actions may be taken on the negativepulse to help ensure that the surface of the glass does not charge up tothe plasma potential or higher. However, in some cases, an acceptablestrength may be achieved with an implantation depth of from about 5 to15 microns.

In certain example embodiments, the chemical strengthening according toexample embodiments described herein may increase the strength of theresulting glass substrate to at least about 200 MPa, more preferably atleast about 400 MPa, and most at least about 600 MPa. In other exampleembodiments, the strength of a resulting glass substrate that ischemically tempered utilizing plasma may have a strength of from about200 to 800 MPa, more preferably from about 400 to 600 MPa, and mostpreferably a strength of at least about 400 MPa, in different exampleembodiments.

This increase in strength may be accomplished in from about 30 secondsto 30 minutes, more preferably about 1 to 20 minutes, and mostpreferably in about 2 to 10 minutes, with an example timeframe ofapproximately 5 minutes. However, the glass may be strengthened for moreor less time according to different example embodiments.

The dynamic fluence or flux may be adjusted by the linear speed of thesubstrate in some examples.

In other example embodiments, the plasma torch may be used to create agradient in a surface region of a glass substrate. In some cases, byaltering the type of ion (e.g., by altering the type of salt, sulfate,other precursor, etc.) injected into the plasma torch, or by alternatingplasma torches with different precursors, a surface region of a glasssubstrate at a first depth may be treated so as to include a particulartype of replacement ion, and then may be treated with another torchand/or type of precursor so as to include a different type ofreplacement ion at a second lesser depth, and so on. An example of aglass substrate with this type of gradient is illustrated in FIG. 6.

FIG. 6 shows a cross-section of chemically strengthened glass substrate1″ produced in accordance with certain example embodiments. In FIG. 6,interior portion 101″ of substrate 1″ still include sodium ions 11.However, surface and/or near-surface regions 100″ include replacementions 10, which may include first replacement ions 3 and secondreplacement ions 19. As can be seen from FIG. 6, first replacement ions3 (which may be Li⁺, K⁺, Mg²⁺, and/or the like) were first driven intothe surface region of substrate 1″, and have replaced some sodium ions.Then, after first replacement ions were driven into the surface region,second replacement ions 19 were driven into the surface region. Thus,substrate 1″ has a gradient with respect to replacement ions. Thisgradient may contain as many internal “sub-layers” as desired, accordingto different example embodiments. Moreover, the gradient may only becreated in one major surface region of the glass substrate. Further, thenumber of internal sub-layers and the composition of those sub-layersmay vary for the different surfaces of the substrate in someembodiments. In certain exemplary embodiments, chemically strengtheninga glass substrate so as to include a gradient may be particularlyadvantageous for curved substrates. However, a gradient may be includedin substantially planar substrates, as well. In certain exampleembodiments, the gradient may be such that particle size decreasestowards the center of the substrate, or vice versa. In other cases, thegradient may vary in other ways, e.g., from large to small to large(etc.), or vice versa.

Another example embodiment relating to chemically strengthening glasssubstrates involves extracting and/or depleting sodium from a moltensoda float line. FIG. 7 illustrates an example of this approach, inwhich the glass ribbon has been altered so as to extract and/or depletesodium from the soda lime silica float glass (e.g., a melt) as it passesthrough (e.g., over) the tin bath. In the FIG. 7 embodiment, the sodiumions extracted and/or depleted from the glass may not be replaced byother ions. Rather, the ions may simply be removed in some cases. Thesodium may be forced out of the float glass/melt, and may be absorbed bythe tin in the tin bath. This may advantageously produce a silicateglass containing less sodium that conventional soda lime silica glass.In certain example embodiments, the silicate glass formed may besubstantially free of sodium (e.g., may contain less than 20 wt. % Na₂O,more preferably less than 15 wt. % Na₂O, still more preferably less than10 wt. % Na₂O, and possibly 1-5 wt. Na₂O). In some embodiments, Argonand/or other gases present in the atmosphere of the float bath may endup in the glass substrate. However, this may not affect the strengthand/or other properties of the glass in certain cases; particularly whenthe gases are inert.

FIG. 7 shows a tin bath portion of a float line in accordance withcertain example embodiment. In certain example embodiments, a dielectricbarrier discharge (DBD) plasma may be set up above a molten glass float(e.g., above a glass ribbon and/or melt) in the tin bath portion of thefloat line. In some cases, the atmosphere may be of or include an inertgas. In certain exemplary examples, an electric field may be set upthrough the float. In cases such as this, the electrodes may be theplasma and the tin bath. In certain examples, a high electric field(e.g., at least about 1000 V/m, more preferably at least about 2000 V/m,and most preferably at least about 5000 or even 10,000 V/m) may be setup in the glass ribbon. This may drive the sodium ions into the tin as asecondary electron emission from the glass. Positive plasma ions mayalso be incorporated into the ribbon to replace the sodium. In certaincases, this may produce a glass that is rich in quartz-like material(s).

More specifically, FIG. 7 illustrates a tin bath 312 in float line 500that acts as an electrode, and plasma 63 that may act as anotherelectrode. In certain example embodiments, the tin bath may be anegative electrode, and the plasma may be a positive electrode 310itself or located proximate to a positive electrode. In some cases, aplasma is struck in the float line, above the tin bath. As the moltenglass ribbon 1000 moves through the tin bath, the tin acts as a negativeelectrode, and the plasma acts as a positive electrode, in certainexample embodiments. Sodium ions are forced out of the glass ribbon, incertain instances. The sodium ions may be absorbed by the molten tin inthe tin bath. In some instances, the sodium ions may be forced out ofthe glass ribbon in a substantially uniform and/or random manner. Inother instances, one side of the glass may have a higher concentrationof sodium ions than the other side. A “sided product” may, for example,be beneficial in certain post-coating applications.

In further example embodiments, the tin bath plasma may be used as a wayof coating a glass substrate with other layers, as well. For example, inaddition to strengthening, the float glass may be coated with layers soas to enhance environmental protection and/or other optical properties.

In certain example embodiments, the electric field may be DC rather thanRF and/or AC, as the tin bath may remain the negative electrode, and theplasma may remain the positive electrode. In other example embodiments,however, other configurations may be possible.

In certain example embodiments, so long as the tin bath and/or floatline comprise a closed chamber so as to contain the argon (and/or otherions in the plasma), the pressure need not be at a vacuum level. In somecases, the pressure may be similar to that typically used in a floatline and/or tin bath (e.g., at or near atmospheric). In otherembodiments, the pressure may be slightly lower. In further cases, itmay be necessary or desirable to control the partial pressure of argonand/or any other replacement ions in the plasma.

In certain example embodiments, the plasma may comprise atoms such asargon. As the sodium ions are driven out of the glass ribbon, the argonatoms 65 may take the place of the sodium ions 11. In certain exampleembodiments, the resulting glass may be a silicate glass; e.g., glasscomprising a silica matrix or silica matrices. In some cases, the glass(e.g., the silica matrix) may include argon. However, argon is inert,and therefore should not significantly adversely affect the propertiesof the glass substrate. In certain example embodiments, a silicate glassformed in this manner may have a strength of at least about 400 MPa,more preferably at least about 600 MPa, even more preferably at leastabout 800 MPa, and in certain exemplary embodiments, the resulting glassmay have a strength of at least about (or even greater than) 1000 MPa.

In certain example embodiments, a level of strength enhancement fromabout 5 to 500 times the strength of unstrengthened glass, sometimesfrom about 7 to 250 times the strength of unstrengthened glass.Typically, it is possible to achieve a strength increase of at leastabout 10 to 100 times compared to the strength of unstrengthened glass.The treatment times necessary to achieve those levels of strengtheningmay be from about 1 second to 1200 seconds, more preferably from about10 to 600 seconds, and most preferably from about 10 to 300 seconds.However, the glass may be strengthened for more or less time accordingto different example embodiments.

In certain example embodiments, the glass transition temperature T_(g)(e.g., the temperature at which the glass reversibly transitions from amolten state to a hard and/or brittle state) may slowly rise as sodiumis extracted. In some cases, this may cause rapid solidification of theribbon. Though rapid solidification may be beneficial in certain exampleembodiments, to be cautious, the power to the float bath and/or ribbonmay be externally increased in certain implementations so as to helpraise the temperature of the float bath and/or ribbon. Further, theheating and cooling may be controlled as the ribbon is drawn. In furtherexample embodiments, alumina-enriched glass may be used. The use ofalumina-enriched glass may also advantageously counteract the rise inT_(g) of the glass melt. That is, the plasma-based strengtheningtechniques of certain example embodiments may be used to createalumina-enriched glass (when Al-inclusive materials are provided in/tothe plasma). Al-enriched glass (e.g., made with a vertical-pulltechnique) produced using the plasma-based techniques disclosed hereinmay have properties and/or characteristics similar to those found inGorilla glass, which is commercially available from Corning. Al-enrichedglass (e.g., made with a float method) produced using the plasma-basedtechniques disclosed herein additionally or alternatively may haveproperties and/or characteristics similar to those found in Dragontailglass, which is commercially available from Asahi. Such glass may beused in electronics-type applications in certain exampleimplementations.

In other example embodiments, the plasma may comprise replacement ionssuch as alkali metals (e.g., Li⁺, K⁺, Mg²⁺, etc.). As the sodium isdriven out of the glass ribbon, the replacement ions in the plasma maytake the place of the sodium ions. The resulting glass substrate wouldhave a strength similar to that of the other methods of ion exchange asdescribed herein. Furthermore, since this is occurring on the floatline, in certain example embodiments, a substantial number of the sodiumions may be replaced by the replacement ions. In other words, a glasssubstrate strengthened in this manner may comprise replacement ionsthroughout its entire thickness, rather than just comprising replacementions in a surface region.

FIG. 8 illustrates a graph of electrode voltage over time that may beused in some of the embodiments described herein. Particularly for ACand/or RF electrodes, FIG. 8 illustrates the voltage of an exampleelectrode as time progresses. In certain example embodiments, theamplitude of an electrode may be from about 10 to 10,000 V, morepreferably from about 50 to 5,000 V, and most preferably from about 100to 1,000 V. In further examples, the corresponding frequency may be fromabout 0.25 KHz to about 500 KHz, more preferably from about 0.5 KHz toabout 100 KHz, and most preferably from about 1 KHz to 50 KHz, accordingto different example embodiments.

In some example embodiments, the voltage may be pulsed. However, inother example embodiments, the voltage may be constant or have othercharacteristics/properties. When pulsed voltage is used, the sheath mayexpand as the pulse is applied. In certain examples, it may beadvantageous to have a bigger sheath in that it may encompass morereplacement ions. FIG. 9 illustrates the pulse. Graph 9 plots voltage v.time, and shows the magnitude of the voltage over time. At the pointcorresponding to 9 a, no voltage has been applied, and there is nosheath around the glass, as can be seen in corresponding glass substrate9 a. At point 9 b, when some voltage has been applied, the sheath 10 bof corresponding substrate 9 b is located very close to the glasssubstrate 9 b. However, as the voltage increases, e.g., as shown atpoint 9 c, the corresponding glass substrate 9 c has sheath 10 c that isyet larger than sheath 10 b. In certain example embodiments, it may beadvantageous to have a larger sheath such as 10 c. In further examples,the fact that the sheath changes size may be advantageous.

It has advantageously been found that since the production methodsproposed herein are available, said production methods may be adapted inmainstream float glass manufacturing processes.

In certain example embodiments, the glass substrates described hereinmay be cleaned and/or polished prior to strengthening.

While the electrodes and/or plasma have been described as being “on” or“supported by” the glass substrate, it will be understood by one skilledin the art that this may mean directly and/or indirectly on or supportedby. While electrodes and/or plasmas may directly contact a glasssubstrate, they may also be spaced apart from the glass substrate indifferent embodiments.

The techniques disclosed herein may be used to strengthen glasssubstrates such as sheets of glass, but may also be used to strengthenother glass articles (e.g., fibers, rods, strips, squares, etc.). Theglass substrates as described herein may be from about 0.01 to severalcentimeters in thickness, more preferably from about 0.01 to 10 mm, andmost preferably less than about 2 mm in thickness.

The example embodiments described herein may be used to strengthen anytype of glass substrate. For example, alumina-enriched glass may bestrengthened by any of the disclosed example methods. Particularly,alumina-enriched glass substrates made by vertical pull methods and/orfloat line methods may be strengthened according to any of the exampleembodiments disclosed herein. In certain example embodiments,alumina-enriched glass substrates may be suitable for use in electronicapplications. In particular, alumina-enriched plasma-strengthened glassmay be particularly advantageous for use in electronic applicationswhere increased durability and reduced thickness are desirable.

Further, although some of the disclosed methods relate to strengtheningsoda lime silica glass by removing and/or replacing sodium ions presentin the glass, it will be appreciated by one skilled in the art that incertain example embodiments, undesirable ions present in a glasssubstrate and/or molten glass ribbon may be advantageously removedand/or replaced utilizing plasma as an electrode and/or replacement ionsource.

While the invention has been described in connection with what ispresently considered to be the most practical and preferred embodiment,it is to be understood that the invention is not to be limited to thedisclosed embodiment, but on the contrary, is intended to cover variousmodifications and equivalent arrangements included within the spirit andscope of the appended claims.

What is claimed is:
 1. A method for increasing the strength of a glasssubstrate, the method comprising: striking a plasma using at least oneplasma source and first and second electrodes disposed on opposing majorsurfaces of a glass substrate, wherein the plasma comprises replacementions; depositing slower ions on a surface of the glass substrate via afirst half-cycle, and then depositing at a faster rate via a secondhalf-cycle, wherein once positive ions fall on the surface of the glasssubstrate, they are driven into the glass substrate by an electric fieldin a sheath, with a high drift speed in order to increase the strengthof the glass substrate, wherein the drift speed is at least about 10m/s.
 2. The method of claim 1, wherein the replacement ions comprise atleast one of Li+, K+ and Mg2+.
 3. The method of claim 1, wherein thereplacement ions replace at least some sodium ions in the glasssubstrate.
 4. The method of claim 1, wherein the glass substrate has astrength of from about 200 to 800 MPa.
 5. The method of claim 4, whereinthe glass substrate reaches said strength in about 1 to 20 minutes. 6.The method of claim 4, wherein the glass substrate reaches said strengthin about 1 to 10 minutes.
 7. The method of claim 1, wherein theelectrodes directly contact the respective major surfaces of the glasssubstrate.
 8. The method of claim 1, wherein the electrodes are spacedapart from the respective major surfaces of the glass substrate.
 9. Themethod of claim 1, wherein the replacement ions penetrate the surfacesof the glass substrate to a depth of at least about 7 microns.
 10. Themethod of claim 1, wherein the replacement ions penetrate the surfacesof the glass substrate to a depth of at least about 15 microns.
 11. Themethod of claim 1, wherein the replacement ions penetrate the surfacesof the glass substrate to a depth of at least about 50 microns.
 12. Themethod of claim 1, wherein the replacement ions substantially replacesodium ions proximate the surface(s) of the glass substrate.
 13. Themethod of claim 1, wherein the glass substrate at least initiallycomprises soda lime silica glass.
 14. The method of claim 1, wherein:the glass substrate is soda lime silica glass; the striking of theplasma provides potassium, lithium, and/or magnesium as replacementions; the driving of the replacement ions into the opposing majorsurfaces replaces sodium ions in surface regions of the glass substrate,the surface regions extending into the glass substrate to depths of atleast about 50 microns; and the glass substrate has a strength of atleast about 400 MPa as a result of the striking and the driving.
 15. Amethod of using plasma to strengthen a glass substrate comprising sodiumions, the method comprising: striking a plasma using at least one plasmasource and first and second electrodes provided over opposing majorsurfaces of a glass substrate, wherein the plasma comprises positiveions; using an electric field set up in connection with the first andsecond electrodes, having a drift speed of at least about 10 m/s, todrive the positive ions into the at least one major surface of the glasssubstrate so as to replace at least some of the sodium ions and increasethe strength of the glass substrate.
 16. The method of claim 15, whereinthe positive ions comprise an alkali metal.
 17. The method of claim 15,wherein the positive ions comprise at least one of potassium, lithiumand magnesium.
 18. The method of claim 15, wherein the electric field isat least about 50 V/mm.
 19. The method of claim 15, wherein the electricfield is at least about 100 V/mm.
 20. The method of claim 15, furthercomprising scanning the electrodes over the surfaces of the substrate inx and/or y directions.
 21. The method of claim 15, wherein the glasssubstrate reaches a strength of at least about 400 MPa in about 1 to 10minutes.
 22. The method of claim 21, wherein the glass substrate reachessaid strength in about 5 minutes or less.
 23. The method of claim 15,wherein the electric field is set up in the substrate.